The present invention relates to the field of hydrocarbon processing.
Many refineries recover valuable products from the heavy residual oil that remains after refining operations are completed. This recovery process, known as delayed coking, produces valuable distillates and coke in one or more large vessels known as coke drums.
Coke drums are typically large, cylindrical vessels having a top head and a frusto-conical bottom portion fitted with a bottom head. Coke drums are usually present in pairs so that they can be operated alternately. Thus, while one coke drum is being filled with residual oil and heated, the other drum is being cooled and purged of up to several hundred tons of coke formed during the previous recovery cycle. The operating conditions of delayed coking can be quite severe. Normal operating pressure typically range from 40 to about 60 pounds per square inch, and the feed input temperature may be over 900° F.
Coke recovery begins with a water quench step in which steam and water are introduced into the coke filled vessel to complete the recovery of volatiles and to cool the mass of coke. The vessel is then vented to atmospheric pressure and the top head (typically a 4-foot diameter flange) is unbolted and removed. A hydraulic coke cutting apparatus is inserted into the vessel to cut the cut the coke, and finally, the bottom head (typically a 7-foot diameter flange) is unbolted and removed to allow the hydraulically cut coke to fall out of the vessel and into a recovery chute. The process of moving the bottom head out of the way of the falling coke is herein referred to interchangeably by the terms deheading and unheading.
There are conceptually only two ways to move the bottom head out of the way of the falling coke. The first way is to completely remove the head from the vessel, perhaps carrying it away from the vessel on a cart. This process may be automated as set forth in U.S. Pat. No. 5,336,375, filed Dec. 15, 1993, entitled “Delayed Coker Drumhead Handling Apparatus,” which is incorporated herein by reference. The other way of “removing” the bottom head is to swing it out of the way, as on a hinge or pivot, while the head is still coupled to the vessel. This process may also be automated, as set forth in Antalffy, et al., U.S. Pat. No. 5,098,524, filed Jul. 29, 1988, entitled “Coke Drum Unheading Device,” commonly assigned with this application, and in the paragraph entitled “Closure Apparatus Application Example” of U.S. Pat. No. 5,048,876 issued Sep. 17, 1991, entitled “Closure Apparatus for Pipes and Vessels,” each of which is also incorporated herein by reference.
Both complete and hinged removal of the head have advantages and disadvantages. Complete removal is advantageous in that it leaves ample room for the discharge of coke, but may require additional floor space, and may be more complicated and costly. Hinged removal is advantageous in that it may be more compact, simpler and more cost effective, but it may not be feasible where the bottom headroom is less than the diameter of the bottom head. In some instances, for example, it may be possible to raise the entire coking vessel, or to cut a hemispherical path for the head out of the adjacent floor, but both of these solutions may be impractical.
Thus, there is a further need for a method and system of deheading delayed coker vessels where the bottom head has less headroom than the diameter of the head. Other and further objects and advantages will appear hereinafter.